Plasma injectors (torches) are primarily used for the purpose of processing refractive materials, which may be either in a liquid or a solid state, and for converting them into vapors which are then ionized by an arc to generate a plasma. The process of vaporizing liquid or solid refractive materials, however, should take place at very high temperatures (&gt;3000K) in order to avoid excessive radiation power losses. In order to achieve such temperatures, the process of vaporizing and ionizing refractive materials is typically started by first ionizing a more volatile gas. A consequence of this is the resultant plasma will include gas ions along with the ions that have been created from the refractive materials.
For applications wherein it is desirable that the plasma include as many refractive material ions as possible in a relatively low density plasma, e.g. densities where the ratio of ion collisional frequency (.nu.) to the cyclotron frequency (.OMEGA.) is greater than one (.nu./.OMEGA.&gt;1), it is necessary for there to be a relatively low gas through-put.
One well known type of plasma torch is the so-called cylindrical ICP Inductively Coupled Plasma), or TCP (Transformer Coupled Plasma) injector. or the cylindrical ICP injector, a generally azimuthal electric field is induced in a cylindrical chamber by a solenoid coil. A gas is then provided in the chamber to initiate the plasma. The strength the electric field induced by the ICP injector, however, has its maximum value at the wall of the chamber and declines toward the center of the chamber. Thus, most of the plasma is created at the wall of the ICP injector. The consequences of this are that very high heat loads are experienced at the wall of the ICP injector and resultant instabilities can be experienced in the chamber. These instabilities are further aggravated by the fact that the solenoid coil also produces a non-axisymmetric electrostatic potential which prevents the induced electric field from being effectively azimuthal. To overcome these adverse consequences, a high gas through-put is introduced near the wall of an ICP injector with angular momentum. This serves the purposes of: 1) initiating and maintaining the plasma, 2) cooling the chamber wall, and 3) helping to stabilize the discharge. As indicated above, however, a high gas through-put for a plasma injector may not always be operationally desirable.
An alternative to the cylindrical ICP injector discussed above is the so-called planar ICP injector which has been widely used for plasma processing semiconductors. The planar ICP injector, unlike the cylindrical ICP injector, is characterized by a planar spiral coil antenna which is placed outside a cylindrical conducting vacuum vessel. The diameter of the antenna can be anywhere between about one half and two thirds of the diameter of the vacuum vessel and the antenna is positioned so that the electromagnetic field it generates will penetrate into the vacuum vessel through a dielectric window. With this configuration, an azimuthal electric field is created in the vessel which has its maximum field strength at the edge of the antenna, and which vanishes at the vessel wall. Interestingly, it happens that the shape of this electric field is similar in several respects to a transverse electric mode (TE) that can be generated by a resonant microwave.
When considering the use of microwaves for the purpose of generating an azimuthal electric field that can generate a plasma, it is necessary to evaluate the thickness of the region in which a plasma can be generated (i.e. the so-called "skin depth"). It happens that at microwave frequencies the skin depth becomes relatively small. Nevertheless, by way of example, if a microwave frequency is taken to be approximately 2.45 GHz, and the plasma conductivity is approximately one Siemens (corresponding to the degree of ionization of 10.sup.-6), the skin depth will be around one cm. Operationally, this value is typical for the higher density plasma torches discussed above.
For the generation of a TE mode electrical field using microwaves, consider a cylindrical cavity having a radius a and a longitudinal axis extending in the z direction. Further, consider the cavity is loaded with a dielectric material which extends from z=-h to z=O, and that it has a conducting end plate at z=-h. In the absence of a plasma in the waveguide, the electric and magnetic fields in the dielectric material in the cavity are given by: EQU E.sub.0 EJ.sub.I [Ir]cos[kz+.phi.]cos .omega.t EQU B.sub.r =-[k/.omega.]EJ.sub.I [Ir]sin[kz+.phi.]sin .omega.t EQU B.sub.z =-[I/.omega.]EJ.sub.0 [Ir]cos[kz+.phi.]sin .omega.t EQU I.sup.2 +k.sup.2 =.epsilon.'.omega..sup.2 /c.sup.2.ident..epsilon.'k.sub.0.sup.2 EQU -kh+.phi.=.pi./2
In the vacuum region of the waveguide, where z.gtoreq.0, the electric and magnetic fields are given by: EQU E.sub..theta. '=E'J.sub.I [Ir]exp[-k'z]cos .omega.t EQU B.sub.r '=-[k'/.omega.]E'J.sub.I [Ir] exp[-k'z]sin .omega.t EQU B.sub.z '=-[I/.omega.]E'J.sub.0 [Ir]exp[-k'z]sin .omega.t EQU k'.sup.2 =I.sup.2 -k.sub.0.sup.2
By equating the fields at the interface we obtain: EQU tan .phi.=k'/k
and EQU E'=E cos .phi.
If the microwave that is generated in the cavity has a cut-off wavelength which corresponds to the lowest frequency that can be supported by the waveguide for the TE mode then, k'=0 and .phi.=0. In this case, the length of the dielectric loaded cavity is a quarter wavelength. If the wave frequency is slightly below cut-off, however, the length of the loaded cavity is a bit shorter than the quarter wavelength.
When the plasma discharge is started in the waveguide, the region z.gtoreq.0 is filled with plasma. By treating the plasma as a conducting medium with the conductivity .sigma., the electric and magnetic fields are given by EQU E.sub..theta. "=E"J.sub.I [Ir]exp[-k"z]cos[k"z+.psi.]cos .omega.t EQU B.sub.r "=[.kappa./.omega.]E"J.sub.I [Ir]exp[-.kappa.z]{cos[.kappa.z+.psi.]+sin[.kappa.z+.psi.]}sin .omega.t EQU B.sub.z "=-[I/.omega.]E"[Ir]exp[-.kappa.z]cos[.kappa.z+.omega.]sin .omega.t EQU .kappa..sup.2 =.mu..sub.0.omega..sigma./2
By matching the fields at the interface for the case where there is plasma in the waveguide, we obtain EQU tan .phi.=-[.kappa./k][1+tan .omega.] EQU E cos .phi.=E" cos .omega.
For the condition where plasma is present in the waveguide, if the skin depth is small, namely .kappa.&gt;&gt;k; .phi..fwdarw.-.pi./2, the cavity length should approach approximately a half wavelength.
Due to the change in conditions that is caused by generating a plasma in the waveguide, it is clear from the above equations that it is necessary to somehow tune the cavity from approximately a quarter wavelength to approximately a half wavelength. It is known that this can be done in several ways. For one, the cavity can be tuned by changing the length of the cavity. Also, the cavity can be tuned by introducing water either into the cavity or with the refractive materials that are being vaporized to create the plasma.
In light of the above it is an object of the present invention to provide a plasma injector which generates an azimuthal TE electric field using microwave power. Another object of the present invention is to provide a plasma injector (torch) which is operational with a low gas through-put. Still another object of the present invention is to provide a plasma injector which will generate a stable discharge. Yet another object of the present invention is to provide a plasma injector which uses an axisymmetric electric field for the generation of a plasma. Another object of the present invention is to provide a plasma injector which is simple to use, is relatively easy to manufacture, and is comparatively cost effective.